Australia
July 29, 2013
Growers are being warned to be on the lookout for diamondback moth (DBM) larvae that are starting to be reported in crops around the Esperance area.
As weather conditions warm, numbers of these pests can blow out and pose a serious challenge to canola crops.
The Grains Research and Development Corporation (GRDC) has issued a new Diamondback Moth Fact Sheet outlining strategies to identify and control outbreaks.
It can be found at www.grdc.com.au/GRDC-FS-DBM and was included in the July-August edition of the GRDC magazine Ground Cover.
Department of Agriculture and Food WA (DAFWA) entomologist Svetlana Micic said DBM larvae had appeared early this year on the eastern south coast and crops in this area should be closely monitored.
DBM larvae grow to 12mm long, are pale yellowish-green and tapered at both ends.
They can usually be found on the underside of leaves and often cause holes that leave the leaf surface intact – creating a window like effect.
Monitoring of numbers of DBM can be done using a sweepnet at the first signs of damage and at intervals during the growing season from mid-July through to late spring and early summer.
“Two sets of sweep samples – each with 10 sweeps of the net – should be carried out at a minimum of four separate locations within each canola crop to get an estimate of numbers and how they vary across paddocks,” Ms Micic said.
“Empty the contents of each sweep on to a white or light coloured surface, such as a vehicle bonnet, and count the number of larvae and presence of other insects.
“If no, or low, numbers of DBM are found, then the crop should be monitored again in two weeks.
“When numbers increase, at least three estimates of larval density across a 12 day period are needed to determine how the population is changing.”
DBM larvae can cause extensive damage to canola crops but there is no ‘one size fits all’ recommendation for control.
Ms Micic said economic thresholds for control varied from 30 to 200 larvae per 10 sweeps and growers needed to consider crop growth stage, grain price and cost of spraying.
She said moisture stressed crops would be more susceptible to damage.
The GRDC Diamondback Moth Fact Sheet guidelines for spraying thresholds are 30 larvae per 10 sweeps at pre-flowering, 50 larvae per 10 sweeps at mid to late-flowering, greater than 50 per 10 sweeps at mid to late flowering and greater than 100 per 10 sweeps at pod maturation.
DBM has developed widespread resistance to many insecticides, including synthetic pyrethroids and organophosphates.
Ms Micic said to reduce the risk of resistance developing to insecticides, it was important that DBM be sprayed only when thresholds were exceeded and that insecticide groups were rotated from one season to the next.
“When sweeping indicates that DBM larval densities are at or above the spray threshold level, then two spray applications of insecticide five to seven days apart are often needed to provide adequate control of larvae and to reduce canola yield losses,” she said.
“The second spray is often needed to control DBM eggs and small caterpillars that survived the first spray.
“Sweep net sampling should be carried out after the first spray to determine its effectiveness and if a second spray is required.
“About 20 per cent of DBM larvae are found on the lower part of the canola canopy, which makes good spray coverage critical.
“Under typical temperature conditions, this will ensure DBM that survive the first application are controlled.”
Ms Micic said it was vital growers check chemical label withholding periods before deciding to spray canola that is nearing windrowing or harvesting and use registered insecticide label rates.
Topics
Species
